The present invention relates to the field of the purification of a gas or gas mixture by adsorption of the impurities which are contained therein on a carbon adsorbent formed by a combination of several different active carbons, in particular a PSA process for purifying a gas, such as hydrogen, nitrogen, oxygen, carbon monoxide, argon, methane or gas mixtures containing them.
A PSA (Pressure Swing Adsorption) unit for purifying a gas usually contains an adsorbent or a combination of adsorbents which has to be capable of selectively retaining the impurities contained in the gas to be treated.
PSA processes and units have proved to be highly effective for separating gas mixtures and especially for obtaining oxygen or nitrogen from air and above all for producing pure hydrogen from gas mixtures contaminated by various impurities.
Now, the production of high-purity hydrogen is of great importance industrially, it being widely used in many synthesis processes such as hydrocracking, methanol production, oxoalcohol production and isomerization processes.
In general, PSA processes benefit from the adsorption selectivity of a given adsorbent for one or more of the contaminating substances in the gas mixture to be purified.
Thus, in the case of hydrogen purification, the impurities that usually have to be removed are: water vapour; CO2, CO, nitrogen, saturated or unsaturated, linear, branched or cyclic hydrocarbons containing one or more carbon atoms in their hydrocarbon structure, for example C1-C8 compounds, such as CH4, C2H4, C2H6, C3H8, BTX (benzene-toluene-xylene) compounds; mercaptans; H2S; SO2, chlorine, ammonia, amines; alcohols, for example C1-C3 light alcohols; other volatile organic compounds, such as esters, ethers and halogenated compounds.
These compounds are generally removed by a number of adsorbents, that is to say layers of adsorbents placed in series. Thus, it is conventional to use alumina or silica gel to retain, in particular, water vapour; activated carbon for retaining, in particular, hydrocarbons, CO2 and water vapour; and zeolite for removing barely adsorbable impurities such as CO and nitrogen.
Usually the adsorbents are placed in a single adsorber but more usually in several adsorbers operating in alternation.
The proportion of the various adsorbents within the composite adsorbent bed depends on the composition of the gas to be treated and on the pressure, and there are therefore many possible combinations of composite adsorbents.
Usually, an H2 PSA unit employs, within each adsorber, a pressure cycle comprising, schematically:
an approximately isobaric production phase at the high pressure of the adsorption cycle;
an adsorbent regeneration phase comprising at least one cocurrent decompression step by pressure equalization with another adsorber; a final, countercurrent depressurization step with discharge of waste gas; and generally an elution step at the low pressure of the cycle, the eluting gas generally coming from a second cocurrent decompression step of an adsorber; and
a repressurization phase comprising at least one step of pressure equalization with another adsorber and a final recompression step by means of production gas.
In general, the cycles may include several, total or partial, equalization steps, preferably from 1 to 4 equalization steps. Gas transfers can take place directly from adsorber to adsorber or via one or more gas storage tanks. The steps of recompression by equalization and of recompression by production gas may or may not be at least partially simultaneous and may optionally include a partial repressurization via a gas feed. Complementary purging steps may be introduced, particularly if it is desired to recover, for reutilization, another fraction other than hydrogen from the gas to be treated. In addition, the cycle may also include standby times during which the adsorbers are isolated.
Conventionally, the adsorption pressure is between 5 bar and 70 bar, preferably between 15 bar and 40 bar; the desorption pressure is between 0.1 bar and 10 bar, preferably between 1 and 5 bar; and the temperature of the stream of hydrogen to be purified is between xe2x88x9225xc2x0 C. and +60xc2x0 C., preferably between +5xc2x0 C. and +35xc2x0 C.
Moreover, this is illustrated, for example, by the documents U.S. Pat. No. 3,702,525, U.S. Pat. No. 3,986,849, U.S. Pat. No. 4,077,779, U.S. Pat. No. 4,153,428, U.S. Pat. No. 4,696,680, U.S. Pat. No. 4,813,980, U.S. Pat. No. 4,963,339, U.S. Pat. No. 3,430,418, U.S. Pat. No. 5,096,470, U.S. Pat. No. 5,133,785, U.S. Pat. No. 5,234,472, U.S. Pat. No. 5,354,346, U.S. Pat. No. 5,294,247 and U.S. Pat. No. 5,505,764, which describe PSA process operating cycles for producing hydrogen.
The impurities are removed by one or more adsorbents placed in series from the upstream end of the adsorber, that is to say the side where the gases to be treated enter the said adsorber. In general, the choice and the proportion of adsorbent(s) to be used depend on the nature or composition of the gas mixture to be treated and on the pressure.
Furthermore, mention may be made of document WO-A-97/45363 which relates to a process for the purification of hydrogen-based gas mixtures polluted by various impurities, including carbon monoxide and at least one other impurity chosen from among carbon dioxide and C1-C8 saturated or unsaturated, linear, branched or cyclic hydrocarbons. The gas stream to be purified is brought into contact, in an adsorption zone, with a first adsorbent selective with respect to carbon dioxide and to C1-C8 hydrocarbons and with a second adsorbent which is a faujasite-type zeolite exchanged to at least 80% with lithium and the Si/Al ratio of which is less than 1.5, in order to remove at least the carbon monoxide (CO). According to this document, the improvement made by the process is due to the use of a particularly effective zeolite, namely an X zeolite exchanged with lithium.
As regards document U.S. Pat. No. 3,150,942, this teaches the use of a zeolite containing sodium cations or sodium and calcium cations in order to purify a stream of hydrogen.
Similarly, document U.S. Pat. No. 4,477,267 describes a process for purifying hydrogen which uses an X zeolite exchanged to from 70 to 90% with calcium cations and also containing an inert binder.
Document U.S. Pat. No. 4,957,514 discloses a process for purifying hydrogen employing an X zeolite exchanged to from 60 to 80% with barium cations.
Furthermore, document U.S. Pat. No. 5,489,327 relates to the purification of gaseous hydrogen by bringing it into contact with a hydride of a zirconium alloy.
Finally, document JP-A-860146024 describes a PSA process for purifying impure gases using a mordenite-type zeolite exchanged with lithium, on the production side, and another zeolite, on the feed side.
On the other hand, certain documents point out that the adsorbent or adsorbents used in a PSA process for purifying hydrogen are of little, or even no importance.
Thus, the document by D. M. Ruthvens, S. Farooq and K. S. Knaebel  less than  less than Pressure Swing Adsorption greater than  greater than , 1994 published by VCH, teaches, on page 238, that  less than  less than since the selectivity for most impurities is high compared with that for hydrogen, any adsorbent can be used greater than  greater than to purify hydrogen.
Similarly, according to document U.S. Pat. No. 4,299,596, any conventional adsorbent can be used to produce hydrogen, for example active carbons, silica gels, molecular sieves, such as zeolites, carbon screens, etc.
Moreover, document U.S. Pat. No. 4,482,361 mentions the possibility of using whatever suitable adsorbent, such as zeolitic molecular sieves, active carbons, silica gels, activated aluminas or similar materials.
Likewise, document U.S. Pat. No. 4,834,780 teaches that the adsorption can be carried out in all cases where an adsorbent has been selected so as to be suitable for the separation process in question, for example active carbons, silica gels, aluminas or molecular sieves.
Moreover, document U.S. Pat. No. 5,275,640 teaches the production of nitrogen from air by a PSA/VSA process with two successive carbon adsorbent beds intended to remove water vapour from the stream of feed air.
Furthermore, document U.S. Pat. No. 5,726,118 proposes a composite adsorbent formed from an intimate mixture of several different active carbons which can be used in a PSA, TSA or VSA process to separate various liquids or gases, especially oxygen and hydrogen.
In general, not one of these documents emphasizes the importance of the choice of active carbon to be used, nor does it emphasize or specify the characteristics that the bed or beds of active carbon must have so that the gas to be treated is purified efficiently.
Based on this, the problem that then arises is how to improve the PSA processes for gas purification, particularly hydrogen purification, that is to say improve the efficiency of the removal of the impurities contained in a stream of gas, such as hydrogen, to be purified.
The invention therefore relates to a process for the purification of a gas stream containing at least one gaseous impurity chosen from the group formed by CO2, saturated or unsaturated, linear, branched or cyclic hydrocarbons containing at least one carbon atom in their hydrocarbon structure, mercaptans, H2S, SO2, chlorine, ammonia, alcohols, amines and volatile organic compounds,
in which the gas stream to be purified is brought successively into contact with particles of a first porous carbon adsorbent and with particles of a second porous carbon adsorbent, the particles of the said first and second porous carbon adsorbents forming separate and successive layers, and
in which the particles of the said first porous carbon adsorbent are defined by a first density (D1), a first specific surface area (SSA1) and a first mean pore size (MPS1) and the particles of the said second porous carbon adsorbent are defined by a second density (D2), a second specific surface area (SSA2) and a second mean pore size (MPS2), such that at least one of the following formulae (1) to (3) is satisfied:
D1 less than D2xe2x80x83xe2x80x83(1) 
SSA1 greater than SSA2xe2x80x83xe2x80x83(2) 
MPS1 greater than MPS2xe2x80x83xe2x80x83(3). 
Depending on the case, the process of the invention may also include one or more of the following characteristics:
the particles of the said first porous carbon adsorbent are defined by a first pore volume (PV1) and the particles of the said second porous carbon adsorbent are defined by a second pore volume (PV2), the first pore volume (PV1) and the second pore volume (PV2) being such that the following formula (4) is satisfied:
PV1 greater than PV2xe2x80x83xe2x80x83(4); 
the particles of the first porous carbon adsorbent and the particles of the second porous carbon adsorbent are inserted within the same adsorber, as separate and successive layers;
the ratio of the volume of particles of the first porous carbon adsorbent to the volume of particles of the second porous carbon adsorbent is between 5/95 and 95/5, preferably between 5/95 and 20/80;
the first and second porous carbon adsorbents are chosen from among active carbons, preferably active carbons produced from coconut husk, peat, lignite, coal, anthracite, polymers or resins. When the first carbon adsorbent is obtained from the same precursor material as the second carbon adsorbent, the said first carbon adsorbent then preferably has a higher degree of activation than the said second carbon adsorbent;
the gas stream is furthermore brought into contact with at least one zeolitic particulate adsorbent, preferably the contacting of the gas stream with the zeolite particles taking place subsequently to the contacting of the said gas stream with the particles of the said first and second porous carbon adsorbents;
the zeolite is a zeolite of the X or A type or a faujasite, exchanged to at least 70% with lithium or with calcium, and/or a zeolite of the faujasite type whose Si/Al ratio is between about 1 and 1.2;
the gas stream is furthermore brought into contact with at least one adsorbent formed from particles of activated alumina or of silica gel, preferably the contacting of the gas stream with the particles of activated alumina or of silica gel taking place prior to the contacting of the said gas stream with the particles of the said first and second porous carbon adsorbents;
it is of the PSA or TSA type and preferably comprises from 2 to 12 adsorbers. Optionally, the adsorber or adsorbers may be regenerated by means of a hot fluid other than the gas to be treated or other than a stream produced in the adsorption unit, such as water vapour;
the gas stream is a hydrogen stream, in particular a synthesis gas coming from the reforming or the cracking of hydrocarbons or of alcohols, such as methanol, or from the gasification of solid carbon products;
the gas stream is a stream of air or nitrogen;
the particles of the first porous carbon adsorbent are defined by: a first pore volume (PV1) where PV1 is between 0.5 and 1.5 cm3/g; a first density (D1) of between 350 and 500 kg/m3; a first specific surface area (SSA1) of between 500 and 1800 m2/g, preferably between 800 and 1500 m2/g; and a first mean pore size (MPS1) where MPS1 greater than 6 xc3xa5ngstrxc3x6ms, preferably between 7 and 15 xc3xa5ngstrxc3x6ms;
the particles of the second porous carbon adsorbent are defined by: a second pore volume (PV2) where PV2 is between 0.4 and 1.3 cm3/g; a second density (D2) of between 400 and 650 kg/g3; a second specific surface area (SSA2) of between 400 and 1500 m2/g, preferably between 600 and 1200 m2/g, and a second mean pore size (MPS2) where MPS2 greater than 4 xc3xa5ngstrxc3x6ms, preferably between 5 and 11 xc3xa5ngstrxc3x6ms;
the particles of the said first and second porous carbon adsorbents have different mean particle sizes, preferably the particle size of the first porous carbon adsorbent is between 2 mm and 5 mm and/or the particle size of the second porous carbon adsorbent is between 1 mm and 3 mm;
the gas is natural gas, a combustion gas or a fermentation gas;
the adsorption pressure is between 5 bar and 70 bar, preferably between 15 bar and 40 bar;
the desorption pressure is between 0.1 bar and 10 bar, preferably between 1 and 5 bar;
the temperature of the stream of hydrogen to be purified is between xe2x88x9225xc2x0 C. and +60xc2x0 C., preferably between +5xc2x0 C. and +35xc2x0 C.;
the porous carbon adsorbent has pores having a size of between 0.4 nm and 4 nm, preferably between 0.5 nm and 2 nm;
it is of the PSA type, having several adsorbers, preferably from 3 to 12 adsorbers;
the stream of gas to be purified furthermore contains water vapour as gaseous impurity.
Now, the inventors of the present invention have shown that, surprisingly, an appreciable improvement in the efficiency of a PSA process for purifying a gas, particularly a stream of hydrogen, of the impurities that it contains can be achieved by judiciously choosing the various layers of carbon adsorbents, that is to say active carbons, used in the PSA process.
This is because, although it is commonplace to use active carbon particles to remove certain impurities contained in hydrogen streams, it has never hitherto been demonstrated that the succession of two microporous active carbons having different pore volumes, different densities, different specific surface areas and/or different mean pore sizes has an appreciable effect on the performance of a PSA-type adsorption process for purifying or separating a gas stream, particularly a hydrogen gas stream.
The active carbons according to the present invention are defined by the method or theory proposed by M. M. Dubunin (see, for example, F. Stoeckli and D. Morel: Chimia 34 (1980) No. 12 (December) or else M. M. Dubinin, Carbon, Vol. 26, No. 1, page 97, (1988).